1. Field of the Invention
This invention is in the area of electronic-ignition systems for internal-combustion engines. In particular, this invention relates to an electronic-ignition system useful in converting the conventional point-contact ignition to electronic ignition.
2. Description of the Prior Art
In recent years, various kinds of electronic-ignition systems have been developed to replace the conventional contact-point ignition system, because the latter, while relatively simple and inexpensive, can be trouble prone. Electronic-ignition systems do away with the problems associated with the contact-point method either by eliminating the points themselves or by reducing the current flow through the points so they last longer. Most late-model American automobiles have some form of electronic-ignition systems, and owners of earlier models often desire to convert the ignition system of their automobile from the conventional point contact to electronic.
One type of electronic-ignition system useful in converting from point-contact ignition to electronic makes use of a magnetic-pulse unit for the trigger mechanism that turns the primary current in the ignition coil on and off. Automobiles have a rotatable shaft located in the distributor, at the end of which is a multisided cam, one side for each cylinder of the engine; conversion to an electronic-ignition system is made in part by mounting a plastic cap over the cam. Inbedded in the cap at equal distance from each other are a plurality of metal inserts of magnetic material, such as iron, with one insert for each cylinder of the engine. As the shaft rotates, the cam and plastic cap thereon and metal inserts therein revolve past a magnetic field, with the metal inserts momentarily disturbing the field at the correct time. Such a disturbance causes formation of a voltage trigger pulse that is amplified and used to control the primary current in the ignition coil.
The plastic-cap approach is useful because it can be mounted directly over the ditributor cam already existing in older model cars, does not require replacement, machining, nor modification of existing cam, any one of which could be prohibitively expensive. Because the plastic cap can be easily mounted over the existing distributor cam, the labor cost for such a conversion is relatively low.
Use of some device, such as the plastic-cap approach, is particularly needed to convert the ignition system of a four-cylinder engine where the distributor cam usually has four sides. As known in the art, the cam functions in conjunction with a rubbing block connected to the contact points to cause the contact points to separate at the correct time, as the shaft rotates. In order to reduce undesirable wear between the corners of the cam and the rubbing block, each of the corners, referred to as lobes, is rounded.
When conversion is made to an electronic-ignition system having a magnetic-pulse unit, the fact that the lobes of the cam are rounded means that even if the material of the cam were magnetic, the shape of the rounded lobes makes the existing cam undesirable by itself for use with the magnetic-pulse unit. Because the amplitude of the trigger pulse generated by a disturbance in the magnetic field is a function of the change in the magnetic field with time, it is essential that the disturbance occur over a very short period of time in order that the amplitude of the pulse be sufficiently high. When the corners of the cam are rounded, rather than at an angle of about 90.degree. or less, the disturbance at slow engine speeds does not occur quickly enough to generate the desired trigger pulse.
By contrast, the metal inserts in a plastic cap as described above are relatively narrow and function when close to the magnetic field to instanteously disturb it, so that the voltage trigger pulse is of sufficient magnitude for normal engine speeds, such as from 500 to 2,500 revolutions per minute.
For maximum applitude of the trigger pulse, the metal inserts in the plastic cap should pass as close as possible to one pole of the magnet creating the magnetic field that the insert is designed to disturb. Preferably, the metal insert should be as close as 0.006 to 0.008 of an inch to the pole. There should be a minimum air gap between the stationary and rotating parts to allow for a margin of safety. However, in order for the plastic cap to hold in place the metal inserts as the shaft and cap rotate rapidly, especially at speeds above 2,500 revolutions per minute, the cap must be sufficiently thick to prevent the metal inserts from flying out of the cap. The thickness of the plastic cap needed to retain the metal inserts while allowing for a minimum air gap results in the metal inserts being further away from the pole than desirable; this, in turn, reduces the trigger voltage pulse, which is undesirable at slow engine speeds, such as around 100 revolutions per minute, which typically is the engine speed during starting.
Thus, there is a need for triggering mechanism to interrupt the magnetic field that is compatible with existing distributor cams, is inexpensive, can fit various sizes of distributor cams, allows the trigger mechanism to be close to a magnetic pole without reducing the air gap below a minimum level, and generates a sufficiently high voltage trigger pulse even during slow engine speeds.